U.S. Pat. No. 5,720,796 of Cheung et al. disclosed grinding aids for treating blast furnace slag feed of roll presses using 0.002 to 0.2 weight percent of polyacrylic acid or its alkali metal salt in combination with up to about 4 weight percent water. Such roll presses are comprised of opposed rollers through which granulated blast furnace slag is fed and ground into finer particles.
The present invention, on the other hand, concerns the grinding of cement clinker which may optionally be interground with other agrillaceous materials such as clay or pozzolanic materials such as natural pozzolan, flyash, limestone, blast furnace slag, and others. The present invention therefore contemplates a different grinding material with different hardness and morphology than the granulated blast furnace slag. In this case, cement clinker is provided in the form of spheroids having an average diameter of 1-3 cm.
The present invention may also be applied to the use of roller mills which employ rollers in an arrangement different from roll presses. Roller mills have sets of grinding rollers that rotate above a grinding table which travels in a circular path, and a classifier that is mounted above the rollers and the grinding table.
Roller mills are designed to pulverize and classify solid materials such as clays, clinker, gypsum, natural pozzolan, limestone, and blast furnace slag. A typical roller mill thus has a set of rollers, a roller table typically ranging from 30 inches to 200 inches in diameter, and a classifier component in one circular housing unit or enclosure. An example is shown in FIG. 1 of U.S. Pat. No. 3,951,347 which is incorporated herein by reference. Roller mills are used for raw materials grinding and finished material grinding in cement manufacturing plants. In the grinding of both raw and finished materials, the materials are conveyed or fed to the center of the roller table and become distributed on the table surface to form a bed or layer of material. Centrifugal force of the unit moves the materials from the center of the table to the outer table rim. As the bed of material passes between the rollers and table, the material becomes progressively pulverized. When the ground materials reach the rim of the grinding table, the lighter and smaller particles are conveyed upwards by an air stream into a cyclone as products, while the heavier and coarser particles drop through the slots and are eventually re-circulated back to the center of the table for re-grinding.
Frequently, the action of the large sets of rollers on the table generates vibrations, and this is said to occur because the bed of material located between the rollers and table do not form a layer having uniform thickness or because a gust of air is generated at the back of the rollers after compaction of the bed materials by the grinding rollers. Many manufacturers have tried to improve the grinding efficiency or bed stability by increasing the frictional forces between the table, material bed, and rollers such as through changes in roller pattern and table design.
One of the objectives of the present invention is to improve the stability and uniformity of the material bed by chemical means, thereby improving the efficiency by which the cement clinker is ground by the rollers.
It is another objective of the present invention to improve the grinding performance, in general, of roller-type mills which employ the stress forces of rollers to comminute cement clinker, alone or in combination with other agrillaceous materials such as clay or pozzolanic materials such as natural pozzolan, flyash, limestone, blast furnace slag or mixture thereof.